A significant number of ultrasonic methods, which provide for imaging of soft tissues of a human body, exist. They are based for example on the Compound Imaging approach, Stereoscopic imaging, Harmonic imaging, the Spatial Compounding Approach, and other methods. These methods are disclosed for example in: U.S. Pat. No. 6,517,487 and in U.S. Patent Applications 2003/0055334, 2003/0055337, 2004/0193047, 2005/0033140, 2005/0049479, 2005/0124886, 2005/0117694, and other patents and applications.
Because of the high attenuation coefficient of a bone compared with a soft tissue, ultrasonic imaging methods known in the art are unsuitable for assessing the skeleton of a human body. Therefore, the conventional ultrasonic imaging systems that are used for studying soft tissue are also not optimized for assessing the skeleton.
Ultrasonic methods for simultaneously quantitative and imaging measurements are known in the art. For example, United States Patent Application 2003/0018263; to Morris, et al; entitled Multi-Zone Transmitter For Quantitative Ultrasound And Image Measurement; filed: Jul. 20, 2001; discloses an ultrasonic transmission unit for an imaging/quantitative ultrasound device provides for coaxial transducer crystals which may be operated independently with a first crystal operated alone for quantitative measurement and the first and second crystal operated together to provide a broad illumination for imaging of structure.
U.S. Patent Application 2005/0107700 proposes a thin film piezoelectric material employing a metallic backer plate to provide high output, non-resonant ultrasonic transmissions suitable for quantitative ultrasonic measurement and/or imaging. Thin film polymer piezoelectric materials such as polyvinylidene fluoride (PDVF) may also be used as an ultrasonic transducer as described in the U.S. Pat. No. 6,305,060 and U.S. Pat. No. 6,012,779.
Ultrasonic investigations of the skeleton can be carried out using the Quantitative UltraSonography (QUS) method. The main ultrasonic parameters evaluated in QUS measurement are the Speed of Sound (SOS) and the frequency dependence of attenuation.
Ultrasonic Attenuation (BUM technique is described by Langton C M, Palmer S B, Porter R W, The Measurement of Broadband Ultrasonic Attenuation in Trabecular Bone, Engineering in Medicine, 13 3, 89-91 (1984). Whilst it now forms the basis of clinical QUS bone assessment, this empirical method is not entirely satisfactory, The BUA technique essentially measures insertion loss, found by comparing the amplitude spectrum of an ultrasonic pulse through bone with the amplitude spectrum of an ultrasonic pulse through a reference medium, typically water. The assumption is made that the attenuation, a (f), is a linear function of frequency, f, between 200-600 kHz. However, a number of authors argued that the assumption of linear relationship does not have any physical basis. (Elinor R Hughes MIOA, Timithy G Leighton FIOA, Graham W Petley, Paul R White “Ultrasonic assessment of bone health” (www.isrt.soton.ac.uk)
U.S. Pat. No. 6,371,916; entitled Acoustic Analysis of Bone Using Point-Source-Like Transducers; to Buhler, et al.; filed: Sep. 3, 1999; discloses an improved apparatus and method for providing a measurement of the characteristic behavior of an acoustic wave in a bone of a subject. A preferred embodiment has first and second transducers and a mounting arrangement for mounting the transducers in spaced relationship with respect to the bone. The first transducer may transmit acoustic energy over a broad solid angle, thereby behaving as a point source of acoustic energy. Additionally or alternatively, the second transducer may collect acoustic energy over a broad solid angle, thereby behaving as a point receiver. A signal processor in communication with the second transducer provides a measurement that is a function of at least one of transient spectral or transient temporal components of the signal received by the second transducer.
U.S. Pat. No. 6,371,916 describes the first invention that proposes an approach for estimation of bone porosity. The invention disclosed in this patent provides a method of determining an index of porosity, which is an indirect qualitative but not quantitative parameter that gives an estimation of porosity and non-connectivity of a bone. However the method does not allow estimates of the value of porosity, images of the distribution of porosity values in a considered volume, or 3D imaging of bones in a human body. It does not provide.
A method for determining porosity value for the inspected media is known from the field of oil geophysics. This method uses a modified Wyllie relationship which can be found in Wyllie, M. R., An experimental investigation of factors affecting elastic wave velocities in porous media, Geophysics, vol. 23, No. 3, 1958. But this approach does not provide values for the porosity distribution in a volume being inspected.
An acoustic energy may travel, reflect, and refract on a boundary between media with different acoustic impedances. Traveling acoustic energy is used in the direct transmission method. For example the U.S. Pat. Nos. 4,926,870; 4,421,119 are described systems, which place a receiver and a transmitter on opposite sides of a bone. The linear propagation of elastic waves is used in the method. This is the conventional method used for bone tissue inspections. The method provides integral estimations. The obtained data is characterized by integral estimations about the travel time from a transmitter to a receiver of longitudinal waves in heterogeneous objects, specifically bones. In the case investigations of heterogeneous object parameters of the object being investigated may increase in one part of an investigated object and decrease in another-part. Significant mistakes in the values of the parameters are therefore obtained because the method is not sensitive to changes in structure of the object and it ignores a heterogeneous property of the object and its anisotropy.
Inspection methods based on refracted energy are used in industry. For example, Time of Flight Diffraction technique (TOFD) is disclosed in U.S. Patent Application 20020134178; to Knight, et al.; entitled Ultrasonic Testing System. In this method a probe with different beam angles is used to detect planar defects by varying an angle of orientation. Usually two probes, one transmitter and one receiver, are arranged on an object's surface. The transmitter sends a relatively wide beam for maximizing the field of the scan. Both probes are aligned geometrically on the same side of an object being studied and A-scans are taken at sequential positions along the length of the object. The data collection on site by the TOFD method is faster than most conventional methods because it uses wide ultrasonic beams for imaging. It is a technique for precise depth assessment. However, this technique does not apply for investigations of anisotropic and heterogeneous materials and is not used in medicine.
Contrary to X-ray CT, Ultrasonic tomography of elastic wave velocities has not found general practical usage in medical imaging. The main problems are deviation of the ultrasound beam due to reflection and refraction caused by gas and bone inclusions in the heterogeneous media.
The use of elastic wave refraction for investigations of bone structure in a human body for medical diagnostics is described in the U.S. Pat. No. 6,221,019 and U.S. Pat. No. 5,143,072. The disclosed methods assume that the refracted waves travel in the field with a boundary between layers with different acoustic impedance, i.e. the boundary between tissue and bone. In the disclosed patents, a transmitter and a receiver are placed on the skin of a patient facing a bone. Ultrasonic waves are transmitted along a transmission path from the transmitter to the bone through the soft tissue surrounding the bone, along the surface of the bone and back through the soft tissue to the receiver at location 1 and location 2. The travel times of the fastest signals between the transmitter and receiver (for location 1 and for location 2) are measured and the acoustic velocity of the bone is calculated based on the distance between the transmitter and the receiver's two locations, the thickness of the soft tissue and the acoustic velocity in the soft tissue. The reflected waves are used for estimations of the acoustic velocity and thickness of soft tissues. The measured parameter, Speed of Sound (SOS) of a cortical bone is calculated from the obtained measured data.
However, this method has the several serious shortcomings. The method is based only on two layers of soft tissue and bone. The method cannot account for anisotropy and the heterogeneous nature of the media of which the bone is comprised and thus produces an integral estimation that does not account for the influence of different parts of an inspected heterogeneous media and therefore leads to significant errors. The method does not map the distribution of changes in a bone by means of an estimated spatial distribution of parameters of the bone's medium. The method does not provide 3D imaging of the volume of the inspected bone. The method suffers an additional inaccuracy and inconvenience caused by the necessity of measuring thickness and longitudinal wave velocity in soft tissues by applying an additional Pulse-Echo method. The method does not estimate the mineral part (matrix) of an inspected bone, porosity values, and porosity distributions in a bone's volume. The method does not provide the locations and estimations of the risk of fracture. The method utilizes high frequency ultrasonic oscillations.